Unilateral sensory-motor stroke can cause significant motor deficits in the arm and leg on the same side of the body as the lesion (ipsilesional), in addition to producing more severe deficits on the opposite side of the body (contralesional). While ipsilesional deficits have been recognized in the clinic for decades, therapeutic attention has understandably focused on the more severe nature of contralesional deficits. However, ipsilesional deficits have recently been shown to substantially limit efficient performance of functional tasks, including activities of daily living. Such limitations are not difficult to understand given that the ipsilesional arm tends to be used as the primary manipulator during both unimanual and bimanual tasks for patients with moderate to severe hemiparesis. These coordination deficits are thought to result from diminished contributions from the damaged hemisphere to control of the arm on the same side of the body, an idea supported by our preliminary studies. Based on the dynamic dominance model of motor lateralization, we hope to explain and predict the differential deficits in unimanual and bimanual coordination that result from either right or left hemisphere damage. The proposed studies exploit the expertise of two laboratories that have invested substantial effort in studying motor lateralization (Sainburg), and ipsilesional motor performance in stroke patients (Haaland). [We expect our results to have tangible applications to rehabilitation, including the development of interventions to improve ipsilesional and bilateral function in chronic stroke patients.] Our proposed experiments examine targeted reaching movements, using a custom designed virtual-reality system that allows real-time display and recording of bilateral arm movements. The first two aims directly examine predictions from our model of lateralization for ipsilesional motor control and learning. Specifically, we will examine whether intersegmental coordination deficits result from left hemisphere damage, while positional deficits result from right hemisphere damage. In addition, we will examine the potential effect of such coordination deficits on functional performance. Our second aim examines whether unilateral stroke produces motor learning deficits that vary with the side of the lesion. Our third and fourth aims address the specific control processes that might underlie ipsilesional deficits: First, we ask whether left hemisphere related deficits in coordination and adaptation are related to errors in predicting task dynamics. Second, we examine whether position errors that result from right hemisphere damage are accounted for by deficient impedance control mechanisms, or rather by deficits in specification of spatial locations. In our final study, we will test our model's predictions for bilateral coordination during virtual object transportation and manipulation tasks. This is particular important for patients with moderate to severe hemiparesis, who often rely on bimanual arm use to carry out activities of daily living. In addition, recent research has indicated that bilateral exercise can serve as an effective therapeutic modality for such patients (Harris-Love et al, 2005). !